Anomalous metallic state and anisotropic multiband superconductivity in Nb3Pd0.7Se7

We report the discovery of superconductivity in Nb3PdxSe7 with an x-dependent superconducting transition temperature as high as T-c similar or equal to 2.1 K for x similar or equal to 0.7 (middle point of the resistive transition). Needlelike single crystals display anisotropic upper-critical fields with an anisotropy gamma = H-c2(b)/H-c2(a) as large as 6 between fields applied along their needle axis (or b axis) or along the a axis. As for the Fe based superconductors gamma is temperature-dependent, suggesting that Nb3Pd0.7Se7 is a multiband superconductor. This is supported by band structure calculations which reveal a Fermi surface composed of quasi-one-dimensional and quasi-two-dimensional sheets of hole character, as well as three-dimensional sheets of both hole and electron character. Remarkably, H-c2(b) is observed to saturate at H-c2(b) (T -> 0 K) similar or equal to 14.1 T which is 4.26 x H-p where H-p is the Pauli-limiting field in the weak-coupling regime. The synthesis procedure yields additional crystals belonging to the Nb2PdxSe5 phase which also becomes superconducting when the fraction of Pd is varied. For both phases we find that superconductivity condenses out of an anomalous metallic state, i.e., displaying partial derivative rho/partial derivative T < 0 above T-c similarly to what is observed in the pseudogap phase of the underdoped cuprates. An anomalous metallic state, low-dimensionality, multiband character, extremely high and anisotropic H-c2's are all ingredients for unconventional superconductivity.